Rolling mill rolls are often made of what is known in the art as a "compound roll". These compound rolls have been made by various manufacturing processes such that the roll in its finished condition has a shell providing the working surface area made of a high alloy material having high hardness and wear resistance and a portion forming the center core and neck sections made of a material with low alloy content and low hardness compared with the shell material. The purpose of producing such a compound roll is to provide a roll which has a high hardness wearing surface while at the same time has neck and center core portions that are resistant to breakage.
It is to be understood that the term "compound roll" as used in this description is considered to refer to all types of rolls which have the above-described dual qualities. In the art, such rolls are referred to by various terms in addition to "compound rolls", such as duplex rolls, double-poured rolls, and differentially heat-treated rolls, as well as other terms.
The most common method of producing compound rolls is by the double-pouring method, which method is also called the dilution, replacement or run-off method. In accordance with this method, a roll mold, which is set to extend along a vertical longitudinal axis, is filled with a high alloy, high hardness metal up to a run-off gate location. After waiting a sufficient length of time to permit solidification of a proper amount of shell thickness, a low alloy, low hardness molten metal is introduced into the mold. Sufficient metal is used to either dilute or replace the necks and center of the roll casting and to fill a shrinkhead portion of the mold above the mold portion forming the upper neck. The shrinkhead portion is filled with the same metal as is used in the dilution process and provides feed metal as the molten casting solidifies and shrinks. After completion of the solidification process, the shrinkhead portion containing a large central void is removed from the casting proper and the result is a compound roll with the proper combination of core and neck breakage resistant qualities in conjunction with the high hardness qualities desired for the wearing surface area.
Another method for making compound rolls is a variation of the above-described method and involves pouring the high hardness shell metal in the same manner as described above and then, after waiting a predetermined amount of time to permit the shell material to solidify, the remaining center portion of the molten metal is drained out and replaced with the low alloy, low hardness material by simply pouring the second metal into the top end of the drained out shell.
Other methods for making compound rolls involve the use of special heat treating processes to provide microstructural differences in the two areas of the roll so that the wanted mechanical properties are produced.
Another method in use today for producing compound rolls involves centrifugal casting.
All of the above-described prior art methods for making compound rolls have several significant disadvantages. The methods involving static casting, namely, dilution, run-off, replacement, etc., require the melting of very substantial amounts of excess metal, and all of these processes, by their very nature, require very precise control in order to produce a satisfactory compound roll. Moreover, for the class of compound rolls using a shell material having a high chromium content (13% or more), excessive amounts of flow through material must be provided. Excessive amounts of flow-through material must be provided in order to decrease the chromium content of the metal which will eventually form the center core and neck portions. Many roll makers aim to reduce the center core material to 1% maximum chromium. This requires more than 180% dilution material. This makes the replacement method very costly.
The centrifugal casting process also requires very precise control in order to achieve satisfactory results and involves the use of equipment that is expensive to obtain and to maintain.
The making of compound rolls by complex differential heat treating methods is very expensive because of the use of special heat treating equipment.